How to mark repeated vertices?

First of all, we have generate a friend network using Nodexl. We want to select one of the friend in the network randomly in order to distribute a information around the whole network.

For example, we double click Peter in the network, then the Graph in the right hand side will highlight all the vertices (Peter's friends) in the vertices worksheet.

Then we want to select Peter's friend(s), let say John, Mary and Kim in order to distribute the information to other friend in the network. By using John, Mary and Kim 's friend network, we can distribute the information over the network.

The problem is it will cover some repeaters who have received the information previously distribute from Peter(Because they have same friends in the network).

So how to mark repeated vertices? Finally,we want to calculate total number of people who received the information.